Such methods and apparatus are known, for example, from U.S. Pat. No. 4,528,957 and U.S. Pat. No. 4,742,808. In both methods, it is indirectly investigated whether the monitored probe exhibits a sufficiently high temperature for reliable lambda value measurements. If this is not the case, the probe is considered not to be operationally ready and the lambda value is not closed-loop controlled but open-loop controlled. Only when the operating temperature is sufficiently high and operational readiness is thus detected, the lambda value is adjusted by closed-loop control.
In the method according to U.S. Pat No. 4,528,957, a counter voltage is connected to the probe voltage via a known resistance and it is monitored whether the resulting voltage leaves a predetermined range, that is whether it exceeds a threshold voltage for lambda values for rich mixtures or drops below a threshold voltage for lambda values for lean mixtures. As long as the voltage is still within the range, an absence of operational readiness is detected and thus the lambda value is open-loop controlled. In the method according to U.S. Pat. No. 4,742,808, the internal resistance of the probe is directly measured, which is done with the aid of a known load resistance and two voltage measurements. As soon as the measured value drops below a resistance threshold value, the probe is considered to be ready for operation.
It has been found in actual operation that, in spite of such measures, it happens that the probe is detected as ready for operation due to shunts even though this is not the case. This results in poor closed-loop control performance which results in high pollutant emission and poor driving quality.
The invention is based on the object of specifying a reliable method for detecting such a fault condition of a lambda probe. The invention is furthermore based on the object of specifying an apparatus for carrying out such a method.